Separation of acetic acid from dioxane by extractive distillation

ABSTRACT

Dioxane cannot be completely removed from dioxane and acetic acid mixtures by distillation because of the presence of the maximum azeotrope. Dioxane can be readily removed from dioxane-acetic acid mixtures by extractive distillation in which the extractive agent is dimethylsulfoxide, either alone or mixed with certain high boiling organic compounds. Examples of effective agents are dimethylsulfoxide; DMSO and octanoic acid; DMSO, hexanoic acid and isophorone.

FIELD OF THE INVENTION

This invention relates to a method for separating acetic acid from dioxane using certain higher boiling liquids as the agent in extractive distillation.

DESCRIPTION OF PRIOR ART

Extractive distillation is the method of separating close boiling compounds or azeotropes by carrying out the distillation in a multiplate rectification column in the presence of an added liquid or liquid mixture, said liquid(s) having a boiling point higher than the compounds being separated. The extractive agent is introduced near the top of the column and flows downward until it reaches the stillpot or reboiler. Its presence on each plate of the rectification column alters the relative volatility of the close boiling compounds in a direction to make the separation on each plate greater and thus require either fewer plates to effect the same separation or make possible a greater degree of separation with the same number of plates. When the compounds to be separated normally form an azeotrope, the proper agents will cause them to boil separately during the extractive distillation and thus make possible a separation in a rectification column that cannot be done at all when no agent is present. The extractive agent should boil higher than any of the close boiling liquids being separated and not form minimum azeotropes with them. Usually the extractive agent is introduced a few plates from the top of the column to insure that none of the extractive agent is carried over with the lowest boiling component. This usually requires that the extractive agent boil twenty Centigrade degrees or more than the lowest boiling component.

At the bottom of a continuous column, the less volatile components of the close boiling mixtures and the extractive agent are continuously removed from the column. The usual methods of separation of these two components are the use of another rectification column, cooling and phase separation or solvent extraction.

Acetic acid, B.P.=118.5° C., and dioxane, B.P.=101.4° C. form a maximum azeotrope boiling at 119.5° C. and containing 77 wt.% acetic acid. When these two are found together in mixtures, either alone or with other liquids, distillation will only produce the azeotrope, never pure acetic acid or dioxane. Thus any liquid mixture containing these two will on distillation produce the azeotrope. Extractive distillation would be an attractive method of effecting the separation of acetic acid from dioxane if agents can be found that (1) will break the acetic acid-dioxane azeotrope and (2) are easy to recover from the acetic acid, that is, form no azeotrope with acetic acid and boil sufficiently above acetic acid to make the separation by rectification possible with only a few theoretical plates.

Extractive distillation typically requires the addition of an equal amount to twice as much extractive agent as the acetic acid-dioxane on each plate of the rectification column. The extractive agent should be heated to about the same temperature as the plate into which it is introduced. Thus extractive distillation imposes an additional heat requirement on the column as well as somewhat larger plates. However this is less than the increase occasioned by the additional agents required if the separation is done by azeotropic distillation. Another consideration in the selection of the extractive distillation agent is its recovery from the bottoms product. The usual method is by rectification in another column. In order to keep the cost of this operation to a minimum, an appreciable boiling point difference between the compound being separated and the extractive agent is desirable. It is desirable that the extractive agent be miscible with acetic acid otherwise it will form a two-phase azeotrope with the acetic acid in the recovery column and some other method of separation will have to be employed.

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method of extractive distillation that will enhance the relative volatility of dioxane to acetic acid in their separation in a rectification column. It is a further object of this invention to identify suitable extractive distillation agents which will eliminate the dioxane-acetic acid azeotrope and make possible the production of pure dioxane and acetic acid by rectification. It is a further object of this invention to identify organic compounds which are stable, can be separated from acetic acid by rectification with relatively few plates and can be recycled to the extractive distillation column and reused with little decomposition.

SUMMARY OF THE INVENTION

The objects of the invention are provided by a process for separating dioxane from acetic acid which entails the use of certain oxygenated or sulfur containing organic compounds as the agent in extractive distillation.

DETAILED DESCRIPTION OF THE INVENTION

We have discovered that dimethylsulfoxide (DMSO), either alone or admixed with other high boiling organic compounds, will effectively negate the dioxane-acetic acid maximum azeotrope and permit the separation of dioxane from acetic acid by rectification when employed as the agent in extractive distillation. Table 1 lists DMSO and its mixtures and the approximate proportions that we have found to be effective. Table 2 lists a few combinations of DMSO which proved to ineffective in spite of their similarity to the mixtures in Table 1. The data in Table 1 was obtained in a vapor-liquid equilibrium still.

                  TABLE 1                                                          ______________________________________                                         Extractive Distillation Agents Which Are Effective In                          Breaking the Acetic Acid - Dioxane Azeotrope.                                                                  Relative                                       Compounds            Ratios     Volatility                                     ______________________________________                                         Dimethylsulfoxide (DMSO)                                                                            1      6/5     3.7 5.5                                    DMSO, Hexanoic acid  (1/2).sup.2                                                                           (3/5).sup.2                                                                            2.4 1.9                                    DMSO, Heptanoic acid "      "       2.2 2.2                                    DMSO, Octanoic acid  "      "       2.4 3.4                                    DMSO, Pelargonic acid                                                                               "      "       1.9 2.1                                    DMSO, Decanoic acid  "      "       2.1 1.9                                    DMSO, Neodecanoic acid                                                                              "      "       2.1 2.0                                    DMSO, Benzoic acid   "      "       2.0 1.6                                    DMSO, Salicylic acid "      "       2.4 1.3                                    DMSO, Cinnamic acid  "      "       1.5 1.5                                    DMSO, o-Toluic acid  "      "       1.5 1.5                                    DMSO, m-Toluic acid  "      "       1.8 1.4                                    DMSO, p-Toluic acid  "      "       1.7 1.7                                    DMSO, o-Hydroxybenzoic acid                                                                         "      "       1.5 1.2                                    DMSO, p-tert. Butyl benzoic acid                                                                    "      "       2.1 2.3                                    DMSO, Azelaic acid   "      "       1.3 1.6                                    DMSO, Hexanoic acid, Isophorone                                                                     (1/3).sup.3                                                                           (2/5).sup.3                                                                            1.9 2.0                                    DMSO, Heptanoic acid, Methyl                                                                        "      "       1.9 1.8                                    benzoate                                                                       DMSO, Octanoic acid, Ethyl benzoate                                                                 "      "       1.8 1.8                                    DMSO, Pelargonic acid, Acetophenone                                                                 "      "       1.8 1.8                                    DMSO, Decanoic acid, Butyl benzoate                                                                 "      "       1.5 1.8                                    DMSO, Neodecanoic acid, Adiponitrile                                                                "      "       2.0 1.6                                    DMSO, Benzoic acid, Diethylene                                                                      "      "       1.9 1.8                                    glycol diethyl ether                                                           DMSO, Cinnamic acid, Dipropylene                                                                    "      "       1.3 1.4                                    glycol dibenzoate                                                              DMSO, m-Toluic acid, Ethylene glycol                                                                "      "       1.4 1.4                                    diacetate                                                                      DMSO, p-Toluic acid, Glycerol                                                                       "      "       1.7 1.4                                    triacetate                                                                     DMSO, p-Hydroxybenzoic acid,                                                                        "      "       1.9 1.1                                    Diethylene glycol diethyl ether                                                DMSO, p-tert. Butyl benzoic acid,                                                                   "      "       2.9 1.7                                    Benzyl ether                                                                   ______________________________________                                    

In each case, the starting material was the dioxane-acetic acid azeotrope. The ratios are the parts by weight of extractive agent used per part of dioxane-acetic acid azeotrope. The relative volatilities are listed for each of the two ratios employed. The compounds which are effective when used in mixtures with DMSO are hexanoic acid, heptanoic acid, octanoic acid, pelargonic acid, decanoic acid, neodecanoic acid, benzoic acid, salicylic acid, cinnamic acid, o-toluic acid, m-toluic acid, p-toluic acid, o-hydroxybenzoic acid, p-tert. butyl benzoic acid, azelaic acid, isophorone, methyl benzoate, ethyl benzoate, acetophenone, butyl benzoate, adiponitrile, diethylene glycol diethyl ether, dipropylene glycol dibenzoate, ethylene glycol diacetate, glycerol triacetate and benzyl ether.

                  TABLE 2                                                          ______________________________________                                         Extractive Distillation Agents Which Are Relatively Ineffective                In Breaking The Acetic Acid - Dioxane Azeotrope.                                                               Relative                                       Compounds            Ratios     Volatility                                     ______________________________________                                         Dimethylsulfoxide (DMSO), Acetyl                                                                    (1/2).sup.2                                                                           (3/5).sup.2                                                                            1.1 1.1                                    salicylic acid                                                                 DMSO, Salicylic acid, Methyl                                                                        (1/3).sup.3                                                                           (2/5).sup.3                                                                            1.0 1.1                                    salicylate                                                                     DMSO, o-Toluic acid, Methyl phenyl                                                                  "      "       1.2 1.3                                    acetate                                                                        DMSO, Azelaic acid, Benzonitrile                                                                    "      "       1.4 1.1                                    ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Data From Run Made In Rectification Column                                                                       Weight                                                                  Weight %                                                                       %      Acetic                                                                               Relative                               Agent     Column    Time   Dioxane                                                                               acid  Volatility                             ______________________________________                                         50% DMSO, Overhead  1 hr.  82     18    1.96                                   50% Pelargonic                                                                           Bottoms          17.3   83.7                                         acid                                                                           ______________________________________                                    

The two relative volatilities shown in Tables 1 and 2 correspond to the two different ratios investigated. For example, in Table 1, one part of DMSO mixed with one part of the dioxane-acetic acid azeotrope gives a relative volatility of 3.7; with 6/5 parts of DMSO, the relative volatility is 5.5. One half part of DMSO mixed with one half part of octanoic acid with one part of the dioxane-acetic acid azeotrope gives a relative volatility of 2.4; 3/5 parts of DMSO plus 3/5 parts of octanoic acid give 3.4. One third part of DMSO plus 1/3 part of hexanoic acid plus 1/3 part of isophorone with one part of the dioxane-acetic acid azeotrope gives a relative volatility of 1.9; with 2/5 parts, these three give a relative volatility of 2.0. In every example in Table 1, the starting material is the dioxane-acetic acid azeotrope which possesses a relative volatility of 1.00.

One of the mixtures, DMSO and pelargonic acid, listed in Table 1 and whose relative volatility had been determined in the vapor-liquid equilibrium still, was then evaluated in a glass perforated plate rectification column possessing 4.5 theoretical plates and the results listed in Table 3. The data in Table 3 was obtained in the following manner. The charge was 250 grams of the dioxane-acetic acid azeotrope and after a half hour of operation in the 4.5 theoretical plate column to establish equilibrium, 50% DMSO and 50% pelargonic acid at 95° C. and 20 ml/min. was pumped in. The rectification was continued with sampling of the overhead and bottoms after one hour. The analysis is shown in Table 3 and was 82% dioxane, 18% acetic acid in the overhead and 17.3% dioxane, 83.7% acetic acid in the bottoms which gives a relative volatility of 1.96 of dioxane to acetic acid. This indicates that the maximum azeotrope has been negated and separation accomplished. Without the extractive agent, the overhead would have been the maximum azeotrope composition of 23% dioxane. This proves that the extractive agent is negating the azeotrope and makes the rectification proceed as if the azeotrope no longer existed and brings out the more volatile dioxane, as overhead.

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 1, 2 and 3. All of the successful extractive distillation agents show that dioxane and acetic acid can be separated from their maximum azeotrope by means of distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable. Without these extractive distillation agents, no improvement above the azeotrope composition will occur in the rectification column. The data also show that the most attractive agents will operate at a boilup rate low enough to make this a useful and efficient method of recovering high purity dioxane and acetic acid from any mixture of these two including the maximum azeotrope. The stability of the compounds used and the boiling point difference is such that complete recovery and recycle is obtainable by a simple distillation and the amount required for make-up is small.

WORKING EXAMPLES Example 1

Fifty grams of the dioxane-acetic acid azeotrope and 50 grams of dimethylsulfoxide (DMSO) were charged to a vapor-liquid equilibrium still and refluxed for 15 hours. Analysis indicated a vapor composition of 37.5% dioxane, 62.5% acetic acid, a liquid composition of 13.9% dioxane, 86.1% acetic acid which is a relative volatility of 3.7. Ten grams of DMSO were added and refluxing continued for another eleven hours. Analysis indicated a vapor composition of 42.9% dioxane, 57.1% acetic acid, a liquid composition of 11.9% dioxane, 88.1% acetic acid which is relative volatility of 5.5.

Example 2

Fifty grams of the dioxane-acetic acid azeotrope, 25 grams of DMSO and 25 grams of octanoic acid were charged to the vapor-liquid equilibrium still and refluxed for 12 hours. Analysis indicated a vapor composition of 28% dioxane, 72% acetic acid and a liquid composition of 13.8% dioxane, 86.2% acetic acid which is a relative volatility of 2.4. Five grams of DMSO and five grams of octanoic acid were added and refluxing continued for another seven hours. Analysis indicated a vapor composition of 34% dioxane, 66% acetic acid and a liquid composition of 15% dioxane, 85% acetic acid which is a relative volatility of 3.4.

Example 3

Fifty grams of the dioxane-acetic acid azeotrope, 17 grams of DMSO, 17 grams of hexanoic acid and 17 grams of isophorone were charged to the vapor-liquid equilibrium still and refluxed for 12 hours. Analysis indicated a vapor composition of 26.1% dioxane, 73.9% acetic acid and a liquid composition of 15.5% dioxane, 84.5% acetic acid which is a relative volatility of 1.9. Three grams each of DMSO, hexanoic acid and isophorone were added and refluxing continued for another 12 hours. Analysis indicated a vapor composition of 26.1% dioxane, 73.9% acetic acid and a liquid composition of 15.1% dioxane, 84.9% acetic acid which is a relative volatility of 2.0.

Example 4

A glss perforated plate rectification column was calibrated with ethylbenzene and p-xylene which possesses a relative volatility of 1.06 and found to have 4.5 theoretical plates. A solution comprising 250 grams of the dioxane-acetic acid azeotrope was placed in the stillpot and heated. When refluxing began, an extractive agent comprising 50% DMSO, 50% pelargonic acid was pumped into the column at a rate of 20 ml/min. The temperature of the extractive agent as it entered the column was 95° C. After establishing the feed rate of the extractive agent, the heat input to the dioxane and acetic acid in the stillpot was adjusted to give a total reflux rate of 10-20 ml/min. After one hour of operation, the overhead and bottoms samples of approximately two ml. were collected and analysed by gas chromatography. The overhead analysis was 82% dioxane, 18% acetic acid. The bottoms analysis was 12.3% dioxane, 83.7% acetic acid. Using these compositions in the Fenske equation, with the number of theoretical plates in the column being 4.5, gave an average relative volatility of 1.96 for each theoretical plate. 

We claim:
 1. A method for recovering dioxane from mixtures of dioxane and acetic acid which comprises distilling a mixture of dioxane and acetic acid in a rectification column in the presence of about one part of an extractive agent per part of dioxane-acetic acid mixture, recovering dioxane as overhead product and obtaining the acetic acid and the extractive agent from the stillpot, wherein said extractive agent comprises dimethylsulfoxide.
 2. The method of claim 1 in which the extractive agent comprises dimethylsulfoxide and at least one member from the group consisting of hexanoic acid, heptanoic acid, octanoic acid, pelargonic acid, decanoic acid, neodecanoic acid, benzoic acid, salicylic acid, cinnamic acid, o-toluic acid, m-toluic acid, p-toluic acid, o-hydroxybenzoic acid, p-tert. butyl benzoic acid, azelaic acid, isophorone, methyl benzoate, ethyl benzoate, butyl benzoate, acetophenone, adiponitrile, diethylene glycol diethyl ether, dipropylene glycol dibenzoate, ethylene glycol diacetate, glycerol triacetate and benzyl ether. 